1. Field of the Invention
The disclosure relates to solar cells and, more particularly, to dilute Group III-V nitride intermediate band solar cells with contact blocking layers to be used for improved solar cell performance.
2. Background Discussion
Solar or photovoltaic cells are semiconductor devices having P-N junctions which directly convert radiant energy of sunlight into electrical energy. Conversion of sunlight into electrical energy involves three major processes: absorption of sunlight into the semiconductor material; generation and separation of positive and negative charges creating a voltage in the solar cell; and collection and transfer of the electrical charges through terminals connected to the semiconductor material. A single depletion region for charge separation typically exists in the P-N junction of each solar cell.
Current traditional solar cells based on single semiconductor material have an intrinsic efficiency limit of approximately 31%. A primary reason for this limit is that a semiconductor has a specific energy gap that can only absorb a certain fraction of the solar spectrum with photon energies ranging from 0.4 to 4 eV. Light with energy below the bandgap of the semiconductor will not be absorbed and converted to electrical power. Light with energy above the bandgap will be absorbed, but electron-hole pairs that are created quickly lose their excess energy above the bandgap in the form of heat. Thus, this energy is not available for conversion to electrical power.
In accordance with one or more embodiments, the dilute III-V nitride p-n junction materials for the IBSC comprise layers of GaNAs, and the contact blocking layers are lattice matched to a desired band gap of the GaNAs layers. In one or more embodiments, the contact blocking layers comprise at least one of AlGaAs or other III-V ternary alloys. In one or more embodiments, the composition of the AlGaAs contact blocking layer is tuned so that its conduction band is aligned with the upper sub-band of the GaNAs absorber layers. By isolating the IBand of the GaNAs absorber layers in this manner and effectively blocking the intermediate band from contact with neighboring layers, the IBand only acts as a “stepping stone” for the absorption of the lower energy photons and hence increases the short circuit current (ISC) of the device. The open circuit voltage (VOC) of a IBSC is determined by the largest gap of the GaNAs. In one or more embodiments, the p-n junction absorber layers of the IBSC may further have compositionally graded nitrogen concentrations to provide an electric field for more efficient charge collection.